"No difference was detected in the distribution of measles antibody or in measles virus in [autism spectrum disorder] cases and controls whether the children had received the first, second, or both MMR vaccinations," Gillian Baird, Ph.D., of Guy's and St Thomas' NHS Foundation Trust, and colleagues wrote online in the Archives of Disease in Childhood.

This study reinforces the findings of two case-control studies published last year and three epidemiological studies published since 1999 that also failed to make any connection between the vaccine and autism spectrum disorders.

Analyzing a cohort of children born from July 1, 1990 through Dec. 31, 1991 in the South Thames region of England, the researchers tested the hypothesis that the MMR vaccine contributed to the pathogenesis of autism spectrum disorders, "as evidenced by signs of a persistent measles infection or abnormally persistent immune response."

Using various diagnostic criteria, Dr. Baird's team recruited 98 children with autism spectrum disorders. They were divided into those with broad disorders (66) and those with narrow autism (32).

The researchers also recruited two control groups. One comprised 52 children with special educational needs but no signs of autism spectrum disorders, and the other, 90 children who were developing normally.

All participants had blood drawn and all had had at least one MMR vaccination.

The researchers tested for a persistent measles infection in peripheral blood mononuclear cells because viral replication occurs here during acute infection and has been reported to contain measles genome in a small number of autistic children. They used three reverse transcriptase PCR assays. Samples were tested to ensure that they were adequate by using a Î²2 microglobulin housekeeping gene PCR with a sensitivity of 10 genome copies per reaction mixture.

All samples were negative for the first two assays. On one of the assays -- the M gene PCR -- three samples were reactive for measles virus, with one sample from the narrow autism group containing the C2 strain and two from the normally developing control group containing the D6 strain. All three samples were negative when retested.

The researchers proposed two explanations for the initial positive finding: laboratory cross-contamination or incomplete immunity, which can allow measles virus to be found in asymptomatic people.

Dr. Baird's team then tested the blood samples for measles IgG antibody with the plaque neutralization test to look for evidence of an abnormal immune response. There was no difference in IgG antibody levels between those with one or two MMR vaccinations (difference=0 log10 (mIU/mL), 95% CI -0.12 to 0.11, P=0.94).

There were no significant differences between the antibody levels in either autism spectrum disorder group or either of the control groups (P=0.13). Differences did not reach statistical significance when those with one or two vaccinations were analyzed separately (P=0.20 and P=0.66, respectively).

The combined control group of special educational needs and normally developing children did not have significantly lower levels of antibodies than those for the narrow autism (P=0.45), broad disorder (P=0.29), or combined autism/broad disorder (P=0.27) groups.

Some level of regression -- either the loss of five or more words used communicatively during a three-month period or a regression of words or skills in communication or play behavior -- was reported in 23 children in the broad disorders group but antibody levels were not significantly higher than those in the combined control group (P=0.18).

The researchers acknowledged limitations to the study, including the fact that the participants in the normal development control group were not randomly selected.

Also, parents were informed that the study was about MMR vaccination, which may have biased those who signed up to participate.

The researchers noted that only 29% of children with a local diagnosis of autism spectrum disorders received a second MMR vaccination, compared with 50% of those without such a diagnosis, a finding that "is of public health relevance. â€¦ This may reflect parental concern about vaccination following a diagnosis of developmental abnormality."

The study was funded by the Department of Health, the Wellcome Trust, the National Alliance for Autism Research, and Remedi.

Dr. Baird has acted as an expert witness for the diagnosis of autism. Two co-authors have given unpaid advice to lawyers, and another has served as an expert witness, in MMR and MR litigation. Another co-author receives royalties from diagnostic tools used in this study.

MedPageToday is a trusted and reliable source for clinical and policy coverage that directly affects the lives and practices of health care professionals.

Physicians and other healthcare professionals may also receive Continuing Medical Education (CME) and Continuing Education (CE) credits at no cost for participating in MedPage Today-hosted educational activities.